Is their a maximun lenght you can run 3″ or 4″ flexible pipe for bathroom fans. Should the pipe be placed so it will be above the insulation? Thanks,
Roger
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Having installed quite a few of these units I have yet to hear any limitations, within reason, on run lengths. As far as I know, Florida doesn't have any. I guess common sense serves because none I have installed has collected any negative comments from the local inspectors.
I have read about there being some recommendation, might be code or just accepted practice, that in some cold climates a set number of feet of run after the fan be insulated to limit condensation and that the outlet end be lower than the run to allow any condensation to drain to the outside.
I think that fantech has a chart. You might do a google for them.
It is a function of the CFMs of the motor and fan and the size of the pipe. Fantech is the one that uses a remote fan for quiet and it can run draft for more than one bath so it can get complicated.
If my memory serves me right it is possible to run up to about 80-90 feet.
This is a little different than a clothes dryer in which a certain amount of heat water vapor (and carbon monoxide in gas dryers) is generated and you MUST exhaust it all.
If the run is too long, the fan will blow less air (like 50 cfm instead of 90 cfm).
The pressure drop through 4" flex is several fold the pressure drop of 4" rigid. So you could go 2 to 3 times further with rigid than with flex. (Considering pressure drop only. Heat loss and possible condensation is another issue. Also worse in flex because more turbulence causes more heat exchange, but not a 2 to 3-fold difference.
3" versus 4" makes a big difference as well. 3" has 4.5 times the pressure drop as 4". And 4" allows twice the volumetric flow rate at the same pressure drop as 3".
In a cold climate (e.g. mine), I keep the exhaust duct within the insualted envelope whenever possible.
Roger,
Do you have a specific fan in mind? The answer to your question is two fold; 1) some codes limit use of flexible duct to a certain length, 2) if the fan develops enough static pressure, the answer is no.
But in reality, an exhaust fan that uses 3" duct is little more than a noise maker. 4" is more common and better, but not much. IMHO, if you are going to the trouble and expense to install a bathroom exhaust fan, get one that will move some air, say 100 cfm or more. For 100 cfm I would recommend you use 6" duct. Insulate it, keep the flex to a minimum.